Strongly acidic cation-exchange resins behave as though they were solid acids in many reactions, and can replace such mineral acids as sulfuric acid and hydrochloric acid as acid catalysts. Because they permit easier product separation, decreased equipment corrosion and expense, and increased product purity, these cation-exchange resins are widely used as catalysts for esterifying acids with alcohols or olefins. For example, see the disclosures in U.S. Pat. Nos. 3,037,052 to Bortnick, 4,332,738 to Benitez et al., 3,278,585 to Baker et al., 3,678,099 to Kemp, 2,678,332 to Cottle and 4,652,406 to Lepper et al. One such esterification reaction, that of inexpensive, naturally occurring palm-oil and coconut-oil fatty acids with alcohols to produce fatty esters, EQU C.sub.n H.sub.m COOH+ROH.fwdarw.C.sub.n H.sub.m COOR+H.sub.2 O
where n=8 to 19 and m=17 to 39, is of considerable interest. These fatty esters may be saturated or unsaturated, and may be used as intermediates for producing surfactants and linear detergent alcohols. Esters of lower molecular weight, where n=1 to 8 and m=3 to 17 have found considerable use as solvents, flavors and fragrances. Another reaction of commercial interest is the esterification of anhydrides with alcohols to produce dialkyl diesters useful in producing diols. Esters of unsaturated acids such as acrylates and methacrylates produced by the esterification of unsaturated acids with alcohols are also of commercial interest. Esters may also be produced by transesterification of esters and alcohols. However, to achieve high organic-acid esterification and transesterification rates when using strongly acidic cation-exchange resins, the water produced by the reaction must be removed, high alcohol concentrations must be used, and the reaction must be conducted at elevated temperatures (e.g. 60.degree. C. to 120.degree. C.). These conditions can produce excellent conversions, but also promote the formation of dialkyl ethers from the acid-catalyzed self condensation of alcohols: EQU ROH+R'OH.fwdarw.R--O--R'+HOH.
The formation of these dialkyl ethers not only wastes the alcohol but also creates problems with product separation and waste disposal.
An approach to reduce the amount of byproduct produced in an esterification reaction is described in U.S. Pat. No. 3,678,099, assigned to Chevron Research Company. In this process isobutene was esterified with a carboxylic acid in the presence of a macroporous, acidic cation-exchange resin having a limited cation-exchange capacity (0.2 to 2.4 meq/g, compared with the usual 4-6 meq/g for fully functionalized, macroporous cation-exchange resins); it had the advantage of reducing the amount of isobutene polymerization. The capacity of this macroreticular resin was reduced by partially neutralizing it with sodium ions.
Although surface-functionalized cation-exchange resins have been produced, as for example by McMaster et al., Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1 (1972), pp. 97-105, who controlled depth of sulfonation to as little as 15% of the total bead diameter by carefully limiting the sulfonation time, and by Widdecke et al., Macromol. Chem. Phys. Suppl., 6 (1984) pp. 211-226, who sulfonated the surface of macroporous resins, such resins have largely remained a laboratory tool for investigating reaction kinetics. Little incentive has existed to use them in industrial processes because of their relatively low cation-exchange capacity, and they are not known as esterification catalysts. Partially sulfonated cation-exchange resins having a cation-exchange capacity between 0.1 and 0.6 meq/g were shown to produce negligible byproducts when used to selectively decompose methyl t-butyl ether to isobutylene and methanol (West German Patent No. DE 3,509,292), and inorganic oxides with modified surfaces have been employed as esterification catalysts for fatty acids (European Patent Application No. EP 310 843), but that reference did not suggest surface functionalization of organic polymer beads with strong-acid functional groups for that purpose.